This invention relates to a second harmonic wave generating device having an extremely high conversion efficiency to second harmonic wave, more specifically, to a second harmonic wave generating device (hereinafter referred to as "SHG device") comprising a LiNbO.sub.3 thin film wave guide formed on a LiTaO.sub.3 substrate.
A SHG device utilizes nonlinear optical effects of a nonlinear optical material to convert wavelength .lambda. of incident laser light to wavelength 1/2.lambda., which is outputted. Since the output light has a half wavelength of incident light, the device can be used in an optical disc memory and CD player to achieve a 4-times increase in recording density, and can be used in a laser printer and photolithography with enhanced resolution.
Heretofore, a bulk single crystal of a nonlinear optical material using a high-output-power gas laser as a light source has been used as a SHG device. However, with recent increases in demand for compact optical disc systems and laser printers and since gas laser requires an external modulator for optical modulation and is not suited for compact design, a SHG device that enables use of a semiconductor laser, which can be directly modulated and is lower in cost and easier to handle than gas laser, has been in demand.
When a semiconductor laser is used as a light source, since the semiconductor laser generally has a low output power of several mW to several ten mW, a SHG device of a thin film waveguide structure which has a particularly high conversion efficiency has been required.
Optical materials that have nonlinear optical effects and can be used in a SHG device include lithium niobate (LiNbO.sub.3), lithium tantalate (LiTaO.sub.3), KTiOPO.sub.4, KNbO.sub.3, Ba.sub.2 NaNb.sub.5 O.sub.15, and Ba.sub.2 LiNb.sub.5 O.sub.15 and, among these, LiNbO.sub.3 is high in nonlinear optical constant and low in optical loss, which makes it most suitable for use in a SHG device.
Heretofore, there have been known methods to form an optical waveguide using LiNbO.sub.3, in which a bulk single crystal of LiNbO.sub.3 is treated by Ti diffusion, proton exchange, or out-diffusion to form layers with different refractive indices. However, waveguides obtained using these methods have problems in that it is difficult to have great differences in refractive index from the bulk crystal (with respect to laser light of less than 1 .mu.m in wavelength), is extremely difficult to make phase matching for the generation of the second harmonic wave, that is, to make the refractive index in the waveguide (or effective refractive index) of incident light conforming to the effective refractive index of the second harmonic wave [Yamada and Miyazaki, IEICE Technical Report, MW87-113 (1988)], and is difficult to obtain a high conversion efficiency because, since the boundary between the waveguide (layer) and the bulk crystal is not well defined, the light wave tends to diffuse from the waveguide and it is thus difficult to concentrate the optical energy.
The inventors, using a LiNbO.sub.3 material and a 0.8 .mu.m-band semiconductor laser light source, have conducted investigations for a combination that enables phase matching between the fundamental light wave and the second harmonic wave, and found a condition that enables phase matching using a structure comprising a LiTaO.sub.3 single crystal having a specific refractive index and a LiNbO.sub.3 thin film as a waveguide layer having a specific refractive index formed on the surface of the LiTaO.sub.3 single crystal, thus accomplishing a previous invention, which was applied for a patent as Japanese Patent Application No. 63-160804/1988.
However, the previous invention was of a type with a very narrow range of application which was able to use only a 0.8 .mu.m-band semiconductor laser as a fundamental laser light source. Furthermore, since refractive index of a substance generally varies with wavelength of light applied, the previous invention could not be applied to laser light sources of different wavelengths.
The inventors have conducted intensive studies using a LiNbO.sub.3 material into a condition for a SHG device which enables phase matching with laser light sources of different wavelenghts. As a result, it has been found that a second harmonic wave generating device comprising a LiNbO.sub.3 thin film waveguide layer formed on a LiTaO.sub.3 substrate can generate a second harmonic wave very efficiently, in which values of an ordinary refractive index (n.sub.oS1) at a fundamental wavelength (.lambda..mu.m) and an extraordinary refractive index (n.sub.eS2) at a second harmonic wavelength (.lambda..mu.m/2) of the LiTaO.sub.3 substrate are individually within specific ranges, and an ordinary refractive index (n.sub.oF1) of the LiNbO.sub.3 thin film waveguide layer at the fundamental wavelength (.lambda..mu.m), an extraordinary refractive index (n.sub.eS2) of the LiTaO.sub.3 substrate at the second harmonic wavelength (.lambda..mu.m/2), and an extraordinary refractive index (n.sub.eF2) of the LiNbO.sub.3 thin film waveguide layer at the second harmonic wavelength (.lambda..mu.m/2) satisfy a specific equation, thus accomplishing the present invention.